Semiconductor Electronics and Communication Systems
What this domain tests
Semiconductor Electronics and Communication Systems tests energy bands, doping, diodes, rectifiers, transistors, logic gates, modulation and communication systems. Expect bias-state and current-gain questions.
Core rules
- Intrinsic semiconductors have equal electrons and holes. n-type doping adds donors (pentavalent); p-type adds acceptors (trivalent).
- A p-n junction diode conducts under forward bias and blocks under reverse bias. Half-wave rectifier uses one diode; full-wave uses two or a bridge.
- Transistor currents: I_E = I_B + I_C. Current gains α = I_C/I_E and β = I_C/I_B; β = α/(1-α).
- Logic gates: OR, AND, NOT, NAND, NOR. NAND and NOR are universal gates.
- Communication: modulation superimposes information on a carrier. AM varies amplitude; FM varies frequency. Bandwidth and antenna size depend on frequency.
Worked example
In a transistor, emitter current is 10 mA and base current is 0.1 mA. Find collector current, α and β. I_C = I_E - I_B = 10 - 0.1 = 9.9 mA. α = I_C/I_E = 9.9/10 = 0.99. β = I_C/I_B = 9.9/0.1 = 99. The reasoning uses the junction relation first, then the definitions; β can also be checked as α/(1-α) = 0.99/0.01 = 99.
Traps
Do not reverse forward and reverse bias. A diode is not an ohmic resistor. Do not add base current to collector current to get emitter current; emitter is the sum. In logic gates, NAND is not AND followed by NOT in name only: its output is low only when all inputs are high. Higher carrier frequency allows smaller antennas but needs wider bandwidth.